Communication control method and user terminal for selecting synchronization reference of device-to-device (D2D)

ABSTRACT

D2D communication is appropriately controlled even in an environment in which a user terminal inside the coverage and a user terminal outside the coverage both exist. A method includes: a step of receiving, by UE  100 - 1  included in a plurality of UEs  100 , from UE  100 -n included in the plurality of UEs  100 , information related to the UE  100 -n; and a step of selecting, by the UE  100 - 1 , a control user terminal for controlling D2D communication, based on the information related to the UE  100 -n, from among the plurality of UEs  100.

TECHNICAL FIELD

The present disclosure relates to a communication control method and auser terminal that are used in a mobile communication system supportingD2D communication.

BACKGROUND ART

In the 3rd Generation Partnership Project (3GPP), which is astandardization project for a mobile communication system, theintroduction of terminal-to-terminal (Device to Device: D2D)communication is considered as a new function launched from the release12 (refer to Non Patent Literature 1).

In D2D communication, direct terminal-to-terminal communication isperformed not via a network, within a terminal group including aplurality of closely-located user terminals. On the other hand, incellular communication, which is normal communication of a mobilecommunication system, a user terminal performs communication via anetwork.

In the D2D communication, since radio communication with lowtransmission power can be performed between closely-located userterminals, the power consumption of the user terminals and the load onthe network can be reduced as compared with the cellular communication.

CITATION LIST Non Patent Literature

Non Patent Literature 1: 3GPP Technical Report “TR 22.803 V12.2.0” June2013

SUMMARY

D2D communication is assumed to be allowed even for a terminal group atleast part of which includes a user terminal outside the coverage of amobile communication system.

Thus, an object of the present disclosure is to provide a communicationcontrol method and a user terminal that can appropriately control D2Dcommunication even in an environment in which a user terminal inside thecoverage and a user terminal outside the coverage both exist.

A first user terminal according to an embodiment comprises: a controllercontaining at least one processor and at least one memory, wherein incase that the first user terminal is out of coverage of a communicationnetwork, the controller is configured to execute processes of: directlyreceiving first device-to-device (D2D) synchronization information froma second user terminal, the first D2D synchronization informationincluding coverage information that indicates whether the second userterminal is in the coverage; determining, based on the coverageinformation, whether the second user terminal is in the coverage; andselecting the second user terminal as a synchronization reference of thefirst user terminal in response to determining that the second userterminal is in the coverage.

A method for performing by a first user terminal according to anembodiment comprises: in case that the first user terminal is out ofcoverage of a communication network, directly receiving firstdevice-to-device (D2D) synchronization information from a second userterminal, the first D2D synchronization information including coverageinformation that indicates whether the second user terminal is in thecoverage; determining, based on the coverage information, whether thesecond user terminal is in the coverage; and selecting the second userterminal as a synchronization reference of the first user terminal inresponse to determining that the second user terminal is in thecoverage.

A device for controlling a first user terminal according to anembodiment comprises: at least one processor and at least one memory,wherein in case that the first user terminal is out of coverage of acommunication network, the at least one processor is configured toexecute processes of: directly receiving first device-to-device (D2D)synchronization information from a second user terminal, the first D2Dsynchronization information including coverage information thatindicates whether the second user terminal is in the coverage;determining, based on the coverage information, whether the second userterminal is in the coverage; and selecting the second user terminal as asynchronization reference of the first user terminal in response todetermining that the second user terminal is in the coverage.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a configuration diagram of a Long Term Evolution (LTE) systemaccording to first and second embodiments.

FIG. 2 is a block diagram of a user equipment (UE) according to thefirst and second embodiments.

FIG. 3 is a block diagram of an evolved Node-B (eNB) according to thefirst and second embodiments.

FIG. 4 is a protocol stack diagram of a radio interface according to thefirst and second embodiments.

FIG. 5 is a configuration diagram of a radio frame according to thefirst and second embodiments.

FIG. 6 is a diagram illustrating D2D communication according to thefirst and second embodiments.

FIG. 7 is a diagram illustrating an operation environment according tothe first embodiment.

FIG. 8 is an operation flowchart according to the first embodiment.

FIG. 9 is a sequence diagram illustrating an operation in an in coveragecase according to the first embodiment.

FIG. 10 is a sequence diagram illustrating a modified example of FIG. 9.

FIG. 11 is a sequence diagram illustrating an operation in an out ofcoverage case according to the first embodiment.

FIG. 12 is a sequence diagram illustrating an operation in a partialcoverage case according to the first embodiment.

FIG. 13 is a sequence diagram illustrating an operation of a UEparticipating in a D2D group according to the second embodiment.

FIG. 14 is a timing chart illustrating a D2D communication operationaccording to the second embodiment.

FIG. 15 is a diagram illustrating respective scenarios on inside andoutside of a coverage according to third and fourth embodiments.

FIG. 16 is a diagram illustrating an operation overview according to thethird embodiment.

FIG. 17 is a sequence diagram illustrating an operation of a D2Dsynchronization source UE according to the third embodiment.

FIG. 18 is a sequence diagram illustrating an operation of a D2Dun-synchronization source UE according to the third embodiment.

FIG. 19 is a diagram illustrating a multi-hop synchronization scheme inan out-of-coverage scenario according to the fourth embodiment.

FIG. 20 is a diagram illustrating a multi-hop synchronization scheme ina partial-coverage scenario according to the fourth embodiment.

FIG. 21 is a diagram illustrating a transfer method of broadcastsynchronization information (D2D synchronization signal (D2DSS) andphysical D2D synchronization channel (PD2DSCH)) in a multi-hopsynchronization scheme according to the fourth embodiment.

FIG. 22 is a diagram illustrating a method for suppressing theinterference between PD2DSCHs according to the fourth embodiment.

DESCRIPTION OF EMBODIMENTS

[Overview of Embodiments]

A communication control method according to first to fourth embodimentsis used in a mobile communication system supporting D2D communicationperformed by a plurality of user terminals. The communication controlmethod includes: a step A of receiving, by a user terminal included inthe plurality of user terminals, from another user terminal included inthe plurality of user terminals, information related to the another userterminal; and a step B of selecting, by the user terminal, a controluser terminal for controlling D2D communication, based on theinformation related to the another user terminal, from among theplurality of user terminals.

In the first and second embodiments, the information related to theanother user terminal is D2D usable resource information related toradio resource usable by the another user terminal in D2D communication.

In the first and second embodiments, in the step B, the user terminalselects the control user terminal based on respective D2D usableresources of the plurality of user terminals, to mitigate interferenceto cellular communication that is caused by D2D communication.

In the first and second embodiments, in the step B, the user terminalselects, from among the plurality of user terminals, a user terminalhaving relatively small radio resource usable in D2D communication, asthe control user terminal.

In the first and second embodiments, in the step A, the user terminalfurther receives, from the another user terminal, coverage relatedinformation as to whether the another user terminal is in coverage ofthe mobile communication system. In the step B, the user terminalselects the control user terminal from among the plurality of userterminals based on the D2D usable resource information and the coveragerelated information.

In the first and second embodiments, in the step A, the user terminalfurther receives, from the another user terminal, power source relatedinformation related to power source used by the another user terminal.In the step B, the user terminal selects the control user terminal fromamong the plurality of user terminals based on the D2D usable resourceinformation, the coverage related information, and the power sourcerelated information.

In the first and second embodiments, at least one of the D2D usableresource information, the coverage related information, and the powersource related information is included in a discovery signal used fordiscovery of neighboring user terminal.

In the first and second embodiments, in the step B, in a case in whichall of the plurality of user terminals is in the coverage, or all of theplurality of user terminals is outside the coverage, the user terminalselects a user terminal having power source of which drivable time isrelatively long, as the control user terminal, from among the pluralityof user terminals.

In the first and second embodiments, the communication control methodfurther includes a step C of transmitting, by the control user terminalselected from among the plurality of user terminals, a broadcast signalfor D2D communication. The broadcast signal includes at least one of: asynchronization signal for synchronizing the plurality of userterminals; D2D usable resource information related to radio resourcesusable by the control user terminal in D2D communication; informationabout allocated resource allocated from among the usable radioresources; and information indicating radio resource usable fortransmitting a request for participation in terminal group.

In the second embodiment, in a case in which a terminal group forperforming D2D communication is not formed, the steps A and B areperformed when the terminal group is formed.

In the second embodiment, in a case in which a terminal group forperforming D2D communication is formed, when a new user terminalparticipates in the terminal group, the steps A and B are performed.

In the second embodiment, in a case in which a terminal group forperforming D2D communication is formed, when it is checked that a partof user terminals forming the terminal group has left, the steps A and Bare performed.

In the second embodiment, the communication control method furtherincludes a step D of controlling, by the user terminal, a transmissiontiming at which a discovery signal is transmitted, and a monitoringtiming at which a discovery signal transmitted from another userterminal is monitored, in a case in which a terminal group forperforming D2D communication is not formed. In the step D, the userterminal controls the transmission timing and the monitoring timingbased on a synchronization signal or a reference signal that istransmitted from a base station, or an operation timing set by the userterminal itself.

In the second embodiment, the communication control method furtherincludes a step of transmitting, by the user terminal, in a case inwhich the discovery signal is received at the monitoring timing, arequest for participation in a terminal group, at a timing that is basedon a reception timing of the discovery signal.

In the second embodiment, first radio resource usable for transmittingthe discovery signal is predefined. The communication control methodfurther includes a step of monitoring, by the user terminal, in a casein which second radio resource usable for transmitting the discoverysignal is allowed from a base station in a case in which a terminalgroup for performing D2D communication is not formed, the discoverysignal at the monitoring timing for each of the first and second radioresources.

In the third and fourth embodiments, the control user terminal is a D2Dsynchronization source. The information related to the another userterminal is priority information of the another user terminal thatindicates a degree of suitability for the D2D synchronization source.

A user terminal according to first to fourth embodiments is included ina plurality of user terminals in a mobile communication systemsupporting D2D communication performed by the plurality of userterminals. The user terminal includes: a receiver configured to receive,from another user terminal included in the plurality of user terminals,information related to the another user terminal; and a controllerconfigured to select a control user terminal for controlling D2Dcommunication, based on the information related to the another userterminal, from among the plurality of user terminals.

A user terminal according to the third embodiments supports D2DProximity Service for enabling direct terminal-to-terminal communicationthat is performed not via a network. The user terminal includes: astorage configured to store D2D resource information indicating radioresource usable for the D2D Proximity Service; and a controllerconfigured to, in a case in which the user terminal itself becomes a D2Dsynchronization source on outside of a coverage of the network, transmitthe D2D resource information stored by the storage, by broadcast. In acase in which the user terminal itself becomes a D2D un-synchronizationsource performing synchronization with the D2D synchronization source,the controller rewrites the D2D resource information stored by thestorage, using the D2D resource information received from the D2Dsynchronization source.

In the third embodiment, the user terminal further includes a receiverconfigured to receive, from another user terminal, priority informationindicating a degree of suitability for the D2D synchronization source.The controller determines whether to set the another user terminal asthe D2D synchronization source or set the user terminal itself as theD2D synchronization source, by comparing the received priorityinformation with predetermined information.

In the third embodiment, the predetermined information is priorityinformation of the user terminal itself or a reference value acquiredfrom the network.

In the third embodiment, the user terminal further includes atransmitter configured to transmit, by broadcast, priority informationof the user terminal itself that indicates a degree of suitability forthe D2D synchronization source. The priority information of the userterminal itself is based on at least one of a specification of the userterminal itself, a movement state of the user terminal itself, andreliability of the D2D resource information stored in the storage.

A method according to the third embodiments is a method in a userterminal supporting D2D Proximity Service for enabling directterminal-to-terminal communication that is performed not via a network.The method includes: a step of storing D2D resource informationindicating radio resource usable for the D2D Proximity Service; a stepof, in a case in which the user terminal itself becomes a D2Dsynchronization source on outside of a coverage of the network,transmitting the D2D resource information stored by the storage, bybroadcast; and in a case in which the user terminal itself becomes a D2Dun-synchronization source performing synchronization with the D2Dsynchronization source, rewriting the D2D resource information stored bythe storage, using the D2D resource information received from the D2Dsynchronization source.

A user terminal according to the fourth embodiment supports D2DProximity Service for enabling direct terminal-to-terminal communicationthat is performed not via a network. The user terminal includes: areceiver configured to receive broadcast synchronization informationmulti-hop transferred from a D2D synchronization source; and acontroller configured to transfer transmission broadcast synchronizationinformation corresponding to the received broadcast synchronizationinformation, to another user terminal The controller applies atransmission parameter different from a transmission parameter appliedto the received broadcast synchronization information, to thetransmission broadcast synchronization information. The transmissionparameter is at least either one of a signal sequence or time andfrequency resources.

In the fourth embodiment, the transmission parameter is associated witha hop number of the broadcast synchronization information from the D2Dsynchronization source. The received broadcast synchronizationinformation includes information about a hop number from the D2Dsynchronization source. The controller applies the transmissionparameter corresponding to the information about the hop number, to thetransmission broadcast synchronization information.

In the fourth embodiment, the controller decides the transmissionparameter to be applied to the transmission broadcast synchronizationinformation, by scanning broadcast synchronization informationtransmitted from another user terminal.

A method according to the fourth embodiment is a method in a userterminal supporting D2D Proximity Service for enabling directterminal-to-terminal communication that is performed not via a network.The method includes: a step of receiving broadcast synchronizationinformation multi-hop transferred from a D2D synchronization source; anda step of performing control to transfer transmission broadcastsynchronization information corresponding to the received broadcastsynchronization information, to another user terminal. The step ofperforming control includes a step of applying a transmission parameterdifferent from a transmission parameter applied to the receivedbroadcast synchronization information, to the transmission broadcastsynchronization information. The transmission parameter is at leasteither one of a signal sequence or time and frequency resources.

[First Embodiment]

An embodiment in which the present disclosure is applied to an LTEsystem will be described below.

(System Configuration)

FIG. 1 is a configuration diagram of an LTE system according to a firstembodiment. As illustrated in FIG. 1, the LTE system according to thefirst embodiment includes UE (User Equipment) 100, E-UTRAN (Evolved-UMTSTerrestrial Radio Access Network) 10, and EPC (Evolved Packet Core) 20.

The UE 100 corresponds to a user terminal. The UE 100 is a mobilecommunication device, which performs radio communication with a cell (aserving cell) formed by the eNB 200. The configuration of the UE 100will be described later.

The E-UTRAN 10 corresponds to a radio access network. The E-UTRAN 10includes eNB 200 (an evolved Node-B). The eNB 200 corresponds to a basestation. The eNBs 200 are connected mutually via an X2 interface. Theconfiguration of the eNB 200 will be described later.

The eNB 200 manages one or a plurality of cells, and performs radiocommunication with the UE 100 that establishes a connection with a cellof the eNB 200. The eNB 200 has a radio resource management (RRM)function, a routing function for user data, a measurement controlfunction for mobility control and scheduling and the like. The “cell” isused as a term indicating a smallest unit of a radio communication area,and is also used as a term indicating a function of performing radiocommunication with the UE 100.

The EPC 20 corresponds to a core network. The EPC 20 includes MME(Mobility Management Entity)/S-GW (Serving-Gateway) 300. The MMEperforms different types of mobility control and the like for the UE100. The S-GW performs transfer control of the user data. The MME/S-GW300 is connected to the eNB 200 via an S1 interface.

FIG. 2 is a block diagram of the UE 100. As illustrated in FIG. 2, theUE 100 includes a plurality of antennas 101, a radio transceiver 110, auser interface 120, a GNSS (Global Navigation Satellite System) receiver130, a battery 140, a memory 150, and a processor 160. The memory 150corresponds to storage, and the processor 160 corresponds to acontroller. The UE 100 may not necessarily have the GNSS receiver 130.Furthermore, the memory 150 may be integrally formed with the processor160, and this set (that is, a chip set) may be called a processor 160′.

The antenna 101 and the radio transceiver 110 are used to transmit andreceive a radio signal. The radio transceiver 110 converts a basebandsignal (a transmission signal) output from the processor 160 into aradio signal and transmits the radio signal from the antenna 101.Furthermore, the radio transceiver 110 converts a radio signal receivedby the antenna 101 into a baseband signal (a reception signal) andoutputs the baseband signal to the processor 160.

The user interface 120 is an interface with a user carrying the UE 100,and includes, for example, a display, a microphone, a speaker, andvarious buttons. The user interface 120 receives an operation from auser and outputs a signal indicating the content of the operation to theprocessor 160. The GNSS receiver 130 receives a GNSS signal in order toobtain location information indicating a geographical location of the UE100 and outputs the received signal to the processor 160. The battery140 accumulates a power to be supplied to each block of the UE 100.

The memory 150 stores a program to be executed by the processor 160 andinformation to be used for processing by the processor 160. Theprocessor 160 includes a baseband processor that performs modulation anddemodulation, encoding and decoding and the like on the baseband signal,and a CPU (Central Processing Unit) that performs various types ofprocesses by executing the program stored in the memory 150. Theprocessor 160 may further include a codec that performs encoding anddecoding on sound and video signals. The processor 160 executes varioustypes of processes and various types of communication protocolsdescribed later.

FIG. 3 is a block diagram of the eNB 200. As illustrated in FIG. 3, theeNB 200 includes a plurality of antennas 201, a radio transceiver 210, anetwork interface 220, a memory 230, and a processor 240.

The antenna 201 and the radio transceiver 210 are used to transmit andreceive a radio signal. The radio transceiver 210 converts a basebandsignal (a transmission signal) output from the processor 240 into aradio signal and transmits the radio signal from the antenna 201.Furthermore, the radio transceiver 210 converts a radio signal receivedby the antenna 201 into a baseband signal (a reception signal) andoutputs the baseband signal to the processor 240.

The network interface 220 is connected to the neighboring eNB 200 viathe X2 interface and is connected to the MME/S-GW 300 via the S1interface. The network interface 220 is used in communication performedon the X2 interface and communication performed on the S1 interface.

The memory 230 stores a program to be executed by the processor 240 andinformation to be used for processing by the processor 240. Theprocessor 240 includes a baseband processor that performs modulation anddemodulation, encoding and decoding and the like on the baseband signaland a CPU that performs various types of processes by executing theprogram stored in the memory 230. The processor 240 executes varioustypes of processes and various types of communication protocolsdescribed later.

FIG. 4 is a protocol stack diagram of a radio interface in the LTEsystem. As illustrated in FIG. 4, the radio interface protocol isclassified into a first layer to a third layer of an OSI referencemodel, such that the first layer is a physical (PHY) layer. The secondlayer includes a MAC (Medium Access Control) layer, an RLC (Radio LinkControl) layer, and a PDCP (Packet Data Convergence Protocol) layer. Thethird layer includes an RRC (Radio Resource Control) layer.

The physical layer performs encoding and decoding, modulation anddemodulation, antenna mapping and demapping, and resource mapping anddemapping. Between the physical layer of the UE 100 and the physicallayer of the eNB 200, user data and control signals are transmitted viaa physical channel

The MAC layer performs priority control of data, a retransmissionprocess by a hybrid ARQ (HARQ), and the like. Between the MAC layer ofthe UE 100 and the MAC layer of the eNB 200, user data and controlsignals are transmitted via a transport channel. The MAC layer of theeNB 200 includes a scheduler for determining (scheduling) a transportformat (a transport block size and a modulation and coding scheme) of anuplink and a downlink, and resource blocks to be assigned to the UE 100.

The RLC layer transmits data to an RLC layer of a reception side byusing the functions of the MAC layer and the physical layer. Between theRLC layer of the UE 100 and the RLC layer of the eNB 200, user data andcontrol signals are transmitted via a logical channel.

The PDCP layer performs header compression and decompression, andencryption and decryption.

The RRC layer is defined only in a control plane that handles controlsignals. Between the RRC layer of the UE 100 and the RRC layer of theeNB 200, a control signal (an RRC message) for various types of settingsis transmitted. The RRC layer controls a logical channel, a transportchannel, and a physical channel according to the establishment,re-establishment, and release of a radio bearer. When there is aconnection (an RRC connection) between the RRC of the UE 100 and the RRCof the eNB 200, the UE 100 is in a connected state (RRC connectedstate). Otherwise, the UE 100 is in an idle state (RRC idle state).

An NAS (Non-Access Stratum) layer positioned above the RRC layerperforms session management, mobility management and the like.

FIG. 5 is a configuration diagram of a radio frame used in the LTEsystem. In the LTE system, OFDMA (Orthogonal Frequency Division MultipleAccess) is applied to a downlink, and SC-FDMA (Single Carrier FrequencyDivision Multiple Access) is applied to an uplink, respectively.

As illustrated in FIG. 5, a radio frame is configured by 10 subframesarranged in a time direction. Each subframe is configured by two slotsarranged in the time direction. Each subframe has a length of 1 ms andeach slot has a length of 0.5 ms. Each subframe includes a plurality ofresource blocks (RBs) in a frequency direction, and a plurality ofsymbols in the time direction. Each resource block includes a pluralityof subcarriers in the frequency direction.

Of the radio resources (time and frequency resources) assigned to the UE100, a frequency resource can be identified by a resource block and atime resource can be identified by a subframe (or a slot).

In the downlink, an interval of several symbols at the head of eachsubframe is a region used as a physical downlink control channel (PDCCH)for mainly transmitting a control signal. Furthermore, the remaininginterval of each subframe is a region available as a physical downlinkshared channel (PDSCH) for mainly transmitting user data.

In the uplink, both ends in the frequency direction of each subframe areregions used as a physical uplink control channel (PUCCH) for mainlytransmitting a control signal. The remaining portion in each subframe isa region available as a physical uplink shared channel (PUSCH) formainly transmitting user data.

(D2D Communication)

The LTE system according to the first embodiment supports D2Dcommunication, which is direct terminal-to-terminal communication(UE-to-UE communication). FIG. 6 is a diagram illustrating D2Dcommunication according to the first embodiment.

The description will now be given based on the comparison between D2Dcommunication and cellular communication, which is normal communicationof the LTE system. The cellular communication is a communication mode inwhich a data path passes through a network (E-UTRAN 10, EPC 20). Thedata path is a transfer path of user data.

On the other hand, as illustrated in FIG. 6, D2D communication is acommunication mode in which a data path set between UEs does not passthrough a network. A plurality of UEs 100 (UEs 100-1 and 100-2) locatedclose to each other directly performs radio communication with lowtransmission power.

Since the plurality of closely-located UEs 100 directly performs radiocommunication with low transmission power in this manner, the powerconsumption of the UEs 100 can be reduced and the interference withadjacent cells can be mitigated, as compared with cellularcommunication. In addition, in first and second embodiments, D2Dcommunication may include a discovery process (Discovery) to bedescribed later.

(Operation According to First Embodiment)

(1) Operation Environment

FIG. 7 is a diagram illustrating an operation environment according tothe first embodiment. In FIG. 7, terminal groups for performing D2Dcommunication (hereinafter, referred to as “D2D groups”) are each formedby two UEs 100. Nevertheless, the number of UEs 100 forming one D2Dgroup may be three or more.

As illustrated in FIG. 7, D2D groups G1 to G3 are formed.

The D2D group G1 is a D2D group formed by UEs 100-11 and 100-12 insidethe coverage of the eNB 200 (hereinafter, briefly referred to as“coverage”). Hereinafter, a case in which UEs 100 forming a D2D groupare each positioned inside the coverage will be referred to as “incoverage”.

The D2D group G2 is a D2D group formed by UEs 100-21 and 100-22 outsidethe coverage. Hereinafter, a case in which UEs 100 forming a D2D groupare each positioned outside the coverage will be referred to as “out ofcoverage”.

The D2D group G3 is a D2D group formed by a UE 100-31 inside thecoverage and a UE 100-32 outside the coverage. Hereinafter, a case inwhich a part of UEs 100 in a D2D group is positioned inside the coverageand a remaining UE 100 is positioned outside the coverage will bereferred to as “partial coverage”.

In this manner, three types of cases are assumed for D2D groups. It istherefore desired that D2D communication can be controlled using amethod common to these three types of cases.

In addition, in a case in which a UE 100 that cannot be controlled bythe eNB 200 exists in a D2D group, as in the out of coverage case andthe partial coverage case, it is desired that a control UE (control userterminal) for controlling D2D communication inside the D2D group isselected, and D2D communication is controlled by the control UE.

The control UE allocates radio resource (frequency, time) to a UE 100included in the D2D group, from among radio resources usable in D2Dcommunication (hereinafter, referred to as “D2D usable resources”).Here, as for a UE 100 inside the coverage, a D2D usable resource isspecified from the eNB 200 to prevent the interference with cellularcommunication. On the other hand, as for a UE 100 outside the coverage,a D2D usable resource is not specified. Thus, all radio resources can beset as D2D usable resources.

In addition, the control UE may relay data transmitted and receivedbetween UEs 100 included in the D2D group. Furthermore, the control UEmay be a synchronization reference (D2D synchronization source) of theUEs 100 included in the D2D group. The first embodiment assumes acommunication mode in which the control UE relays data transmitted andreceived between the UEs 100 included in the D2D group. Thus, thecontrol UE needs to exist also in the “in coverage case”.

(2) Operation Overview

A communication control method according to the first embodiment is usedin a mobile communication system supporting D2D communication performedby a plurality of UEs 100. The communication control method includes astep A of receiving, by a UE 100-1 included in the plurality of UEs 100,from another UE 100-n included in the plurality of UEs 100, D2D usableresource information related to a D2D usable resource of the other UE100-n, and a step B of selecting, by the UE 100-1, a control UE forcontrolling D2D communication, based on the D2D usable resourceinformation, from among the plurality of UEs 100. The D2D usableresource information is an identifier of each of frequency resources(resource block, frequency band, etc.) constituting a D2D usableresource, and/or an identifier of each of time resources (subframe,radio frame, etc.) constituting the D2D usable resource. Alternatively,the D2D usable resource information may be the number of frequencyresources constituting the D2D usable resource, and/or the number oftime resources constituting the D2D usable resource.

With this configuration, when a UE 100 for which a D2D usable resourceis specified is included in the plurality of UEs 100, the control UE canbe selected in consideration of the specified D2D usable resource. Inaddition, in the first embodiment, in the step B, the UE 100-1 selectsthe control UE so as to mitigate the interference to cellularcommunication that is caused by D2D communication. For example, in thestep B, by selecting a UE 100 having a relatively small D2D usableresource, as the control UE, the interference between D2D communicationand cellular communication can be effectively prevented.

In the first embodiment, in the step A, the UE 100-1 further receives,from the other UE 100-n, coverage related information as to whether theother UE 100-n is inside a coverage of the mobile communication system.In the step B, the UE 100 selects the control UE from among theplurality of UEs 100 based on the D2D usable resource information andthe coverage related information. The coverage related information mayinclude a cell identifier of a cell (or an identifier of an eNB) inwhich the other UE 100-n exists.

With this configuration, the control UE can be selected consideringwhich of the “in coverage case”, the “out of coverage case”, and the“partial coverage case” a corresponding case falls into.

In the first embodiment, in the step A, the UE 100-1 further receives,from the other UE 100-n, power source related information related to apower source used by the other UE 100-n. In the step B, the UE 100-1selects the control UE from among the plurality of UEs 100 based on theD2D usable resource information, the coverage related information, andthe power source related information. The power source relatedinformation is information indicating the type of a power source (analternating current (AC) power, a battery, etc.), and/or the amount ofpower that can be supplied from the power source (battery remainingamount, etc.).

With this configuration, the control UE can be selected in considerationof a drivable time defined according to the type and the state of thepower source.

In addition, at least one of the D2D usable resource information, thecoverage related information, and the power source related informationmay be included in a discovery signal used for discovery of aneighboring UE 100 (hereinafter, referred to as “discovery signal”).With this configuration, a process of forming a D2D group and a processof selecting a control UE can partially share a common process. This canenhance the efficiency.

In the first embodiment, in the step B, in a case in which all of theplurality of UEs 100 is inside the coverage, or all of the plurality ofUEs 100 is outside the coverage, the UE 100-1 selects a UE 100 having apower source of which a drivable time is relatively long, as the controlUE, from among the plurality of UEs 100. As a result, a UE 100 with ashort drivable time can be prevented from becoming undrivable (operationstop).

In the first embodiment, the communication control method furtherincludes a step C of transmitting, by the control UE selected from amongthe plurality of UEs 100, a broadcast signal (broadcast information) forD2D communication. The broadcast signal includes D2D usable resourceinformation related to D2D usable resources of the control UE. Each UE100 included in the D2D group performs D2D communication within a rangeof the D2D usable resources of the control UE. Alternatively, thebroadcast signal may include information indicating an allocatedresource allocated from among the usable radio resources. In addition,the broadcast signal may include a synchronization signal for performingsynchronization between the plurality of UEs 100. Furthermore, thebroadcast signal may include information indicating radio resourceusable for transmitting a request for participation in the D2D group.With this configuration, the control UE can appropriately control theD2D group.

(3) Operation Specific Example

Next, a specific example of an operation according to the firstembodiment will be described.

(3.1) Operation Flow

FIG. 8 is an operation flowchart according to the first embodiment.

As illustrated in FIG. 8, in step S11, the UE 100-1 exchangesinformation with a neighboring UE 100 (the UE 100-n). Specifically, D2Dusable resource information, coverage related information, and powersource related information (hereinafter, these pieces of information arecollectively referred to as “D2D UE information”) are exchanged. Inaddition, the D2D UE information includes an identifier of atransmission source UE.

In step S12, the UE 100-1 determines, based on the coverage relatedinformation, whether all of the UEs 100-1 to 100-n are outside thecoverage. When “YES” in step S12, this case falls into the out ofcoverage case.

When “NO” in step S12, in step S13, the UE 100-1 determines, based onthe coverage related information, whether the UEs 100-1 to 100-n existin the same cell (or the eNB 200). In addition, when the cell is dividedinto sectors, the UE 100-1 may determine whether the UEs 100-1 to 100-nexist in the same sector.

In addition, when “YES” in step S13, this case falls into the incoverage case inside the same cell. In addition, when “NO” in step S13,this case falls into the partial coverage case or the in coverage casein difference cells.

When “YES” in step S12 or when “YES” in step S13, in step S14, the UE100-1 sets all of the UEs 100-1 to 100-n as candidates for the controlUE.

On the other hand, when “NO” in step S13, in step S15, based on the D2Dusable resource information, the UE 100-1 sets, from among the UEs 100-1to 100-n, UEs 100 inside the coverage (under the eNB) as candidates forthe control UE.

In step S16, the UE 100-1 selects, from among the candidates for thecontrol UE, a UE 100 with high priority set based on the power sourcerelated information, as the control UE. Specifically, the UE 100-1selects, as the control UE, a UE 100 of which a drivable timecorresponding to the power source related information is relativelylong.

(3.2) Operation in In Coverage Case

FIG. 9 is a sequence diagram illustrating an operation in the incoverage case according to the first embodiment. In FIG. 9, all of UEs100-11 to 100-13 exist in the same cell.

As illustrated in FIG. 9, in step S101, the eNB 200 notifies the UEs100-11 to 100-13 of radio resource “S1” for allowing the use of D2Dcommunication. The notification is performed by broadcast. All of therespective D2D usable resources of the UEs 100-11 to 100-13 are theradio resource “S1”. In addition, after the control UE is once decided,in the D2D group, each UE 100 other than the control UE may notify thecontrol UE of D2D usable resource information, and the control UE maydetermine a usable resource of a corresponding UE 100, and notify thecorresponding UE 100 of the determined usable resource.

In steps S102 to S104, the UEs 100-11 to 100-13 notify one another ofD2D UE information. The notification is performed by broadcast.

In step S105, the UEs 100-11 to 100-13 each select a control UE inaccordance with the above-described operation flow. In this example, allof the UEs 100-11 to 100-13 exist in the same cell, and a drivable timeof a power source (battery) of the UE 100-11 is the longest. Thus, theUE 100-11 is selected as the control UE.

In step S106, the UE 100-11 selected as the control UE transmits abroadcast signal.

In addition, in FIG. 9, the control UE is assumed to serve as asynchronization reference in the D2D group. Nevertheless, in the incoverage case, the eNB 200 may serve as a synchronization reference inthe D2D group. FIG. 10 is a sequence diagram illustrating a modifiedexample of FIG. 9. As illustrated in FIG. 10, steps S111 to S115 aresimilar to corresponding steps in FIG. 9. On the other hand, FIG. 10differs from FIG. 9 in that, in step S116, the eNB 200 transmits asynchronization signal to the UEs 100-11 to 100-13.

(3.3) Operation in Out of Coverage Case

FIG. 11 is a sequence diagram illustrating an operation in the out ofcoverage case according to the first embodiment. In FIG. 11, all of UEs100-21 to 100-23 are outside the coverage.

As illustrated in FIG. 11, in steps S121 to S123, the UEs 100-21 to100-23 notify one another of D2D UE information. The notification isperformed by broadcast.

In step S124, the UEs 100-21 to 100-23 each select a control UE inaccordance with the above-described operation flow. In this example, allof the UEs 100-21 to 100-23 are outside the coverage, and a drivabletime of a power source (battery) of the UE 100-21 is the longest. Thus,the UE 100-21 is selected as the control UE.

In step S125, the UE 100-21 selected as the control UE transmits abroadcast signal.

(3.4) Operation in Partial Coverage Case

FIG. 12 is a sequence diagram illustrating an operation in the partialcoverage case according to the first embodiment. In FIG. 12, the UE100-31 exists in a cell of the eNB 200, and UEs 100-32 and 100-33 areboth outside the coverage.

As illustrated in FIG. 12, in step S131, the eNB 200 notifies the UE100-31 of radio resource “S1” for allowing the use of D2D communication.The notification is performed by broadcast. A D2D usable resource of theUE 100-31 is the radio resource “S1”. On the other hand, each of the UEs100-32 and 100-33 can use all radio resources “S2 (>S1)”.

In steps S132 to S134, the UEs 100-31 to 100-33 notify one another ofD2D UE information. The notification is performed by broadcast.

In step S135, the UEs 100-31 to 100-33 each select a control UE inaccordance with the above-described operation flow. In this example, theD2D usable resource of the UE 100-31 is the smallest. Thus, the UE100-31 is selected as the control UE.

In step S136, the UE 100-31 selected as the control UE transmits abroadcast signal.

(Summing-Up of First Embodiment)

In the first embodiment, by selecting, as a control UE, a UE 100 havinga relatively small D2D usable resource (i.e., a UE 100 for which a D2Dusable resource is specified), the interference between D2Dcommunication and cellular communication can be effectively prevented.

In addition, in the first embodiment, in a case in which all of theplurality of UEs 100 is inside the coverage, or all of the plurality ofUEs 100 is outside the coverage, among the plurality of UEs 100, a UE100 having a power source with a relatively long drivable time isselected as a control UE. With this configuration, a UE 100 with a shortdrivable time can be prevented from becoming undrivable (operationstop).

[Second Embodiment]

The second embodiment will be described mainly based on a differencefrom the first embodiment. A system configuration and an operationenvironment according to the second embodiment are similar to thoseaccording to the first embodiment.

A method for selecting a control UE has been described in the firstembodiment. In the second embodiment, a method for forming and managinga D2D group will be described.

(Operation According to Second Embodiment)

(1) Initial Formation of D2D Group

In the second embodiment, a step D of controlling, by the UE 100 thatstarts D2D communication, a transmission timing at which a discoverysignal is transmitted, and a monitoring timing at which a discoverysignal transmitted from another UE 100-2 is monitored, in a case inwhich a D2D group is not formed is further included. When thetransmission and the monitoring of discovery signals are alternatelyperformed, for example, a discovery signal is transmitted once in onesecond, and a discovery signal is monitored while a discovery signal isnot transmitted.

In the step D, the UE 100 controls the transmission timing and themonitoring timing of discovery signals based on a synchronization signaltransmitted from the eNB 200, or an operation timing set by the UE 100itself. Specifically, in the case of being inside the coverage, the UE100 controls the transmission timing and the monitoring timing ofdiscovery signals based on a synchronization signal transmitted from theeNB 200. On the other hand, the case of being outside the coverage, theUE 100 controls the transmission timing and the monitoring timing ofdiscovery signals based on an operation timing set by the UE 100 itself.

The UE 100 that has received a discovery signal at the monitoring timingtransmits a participation request at a timing that is based on areception timing of the discovery signal. With this configuration,synchronization with the transmission side of the discovery signal canbe achieved. Then, triggered by the transmission and reception of theparticipation request, the above-described operations according to thefirst embodiment (steps A and B) are performed. The participationrequest includes D2D UE information. Alternatively, as described above,the discovery signal may include D2D UE information.

(2) Participation of UE in D2D Group

In the second embodiment, in a case in which a D2D group for performingD2D communication is formed, when a new UE 100 participates in the D2Dgroup, the above-described operations according to the first embodiment(steps A and B) are performed. As a result, when the new UE 100 is moresuitable as a control UE, the new UE 100 can be switched as a controlUE. In addition, there is no need to exchange D2D UE information betweenall the UEs 100 in the D2D group. It is sufficient that D2D UEinformation is exchanged between the new UE 100 and the control UE.

FIG. 13 is a sequence diagram illustrating an operation of a UEparticipating in a D2D group according to the second embodiment. In aninitial state, UEs 100-42 to 100-44 form a D2D group, and the UE 100-42among these UEs is a control UE. In addition, the out of coverage caseis assumed.

As illustrated in FIG. 13, in step S401, the UE 100-41 transmits aparticipation request by broadcast. The participation request includesD2D UE information (D2D usable resource information, coverage relatedinformation, power source related information).

In step S402, the UE 100-42 that has received the participation requesttransmits D2D UE information by broadcast.

In step S403, the UEs 100-41 to 100-44 each select a control UE inaccordance with the above-described operation flow according to thefirst embodiment. In this example, the UE 100-41 has a longer drivabletime than that of the UE 100-42. Thus, the UE 100-41 is selected as thecontrol UE.

In step S404, the UE 100-41 selected as the control UE transmits abroadcast signal.

(3) Checking UE Leaving from D2D Group

In the second embodiment, in a case in which a D2D group for performingD2D communication is formed, when it is checked that a part of UEs 100forming the D2D group has left, the above-described operations accordingto the first embodiment (steps A and B) are performed. As a result, evenin a case in which special connection for D2D communication is not set,it can be checked whether a part of UEs 100 has left.

For example, in a case in which a D2D group for performing D2Dcommunication is formed, the control UE periodically transmits, bybroadcast, a setting request (an update request) of the control UE. TheUE 100 that has received the setting request of the control UE transmitsD2D UE information. The control UE checks whether a part of UEs 100 hasleft, according to the reception status of D2D UE information. Inaddition, by periodically performing a setting request of the controlUE, the control UE can be updated to an appropriate control UE accordingto the situation.

Alternatively, in a case in which a D2D group for performing D2Dcommunication is formed, the control UE monitors a transmissionfrequency of another UE 100 in D2D communication, and transmits, bybroadcast, the setting request (update request) of the control UE basedon the variation degree of the transmission frequency. For example, whenthe control UE detects a UE 100 of which a transmission frequency hasrapidly dropped, the control UE transmits, by broadcast, the settingrequest (update request) of the control UE.

(4) D2D Communication Operation

As described above, the control UE relays data transmitted and receivedbetween UEs 100 included in the D2D group. In addition, the control UEincludes the number of UEs in the D2D group, in a broadcast signal. EachUE 100 decides a transmission timing based on the number of UEs in theD2D group and the order in which a corresponding UE 100 participates inthe D2D group. For example, each UE 100 decides a transmission timingbased on the control UE in the following manner. When there are two UEs,the control UE is in the first place and a UE 2 is in the second placein the priority order. If a new UE participates next, the UE is in thethird place in the priority order. Alternatively, the control UE maydefine the respective orders of UEs 100 and notify the UEs 100 of thedefined respective orders using a broadcast signal.

FIG. 14 is a timing chart illustrating a D2D communication operationaccording to the second embodiment.

As illustrated in FIG. 14, when the number of UEs 100 forming a D2Dgroup is three, a time t0 corresponds to a transmission timing of thecontrol UE, a time t2 corresponds to a transmission timing of a UE 1,and a time t4 corresponds to a transmission timing of a UE 2. Times t1,t3, and t5 correspond to timings at which the control UE relays(retransmits) transmission data of the other UEs. The times t0 and t1both correspond to the transmission timings of the control UE. This timet0 is set as a transmission timing of a broadcast signal(synchronization, resource, the number of UEs, etc.), or as atransmission timing of a participation request of a new UE.

[Modified Example of First and Second Embodiments]

In the above-described first and second embodiments, the eNB 200performs the notification of radio resource for allowing the use of D2Dcommunication, by broadcast. Alternatively, the notification may beperformed by unicast or multicast. In addition, the exchange of D2D UEinformation in the D2D group is performed by broadcast. Alternatively,the exchange may be performed by unicast or multicast.

In the above-described first and second embodiments, D2D UE informationincludes three types of information: D2D usable resource information,coverage related information, and power source related information.Nevertheless, information included in the D2D UE information may be onlyany one of the D2D usable resource information, the coverage relatedinformation, and the power source related information.

In the above-described first and second embodiments, radio resource(frequency, time) usable for transmitting the discovery signal has notbeen mentioned. Nevertheless, radio resource (first radio resource)usable for transmitting the discovery signal may be predefined. Inaddition, when second radio resource usable for transmitting thediscovery signal is separately allowed from the eNB 200, it ispreferable that the UE 100 monitors the discovery signal at themonitoring timing for each of the first and second radio resources.

In the above-described first and second embodiments, an LTE system hasbeen described as an example a mobile communication system. The presentdisclosure, however, is not limited to the LTE system. The presentdisclosure may be applied to a system other than the LTE system.

[Third Embodiment]

The third embodiment will be described mainly based on a difference fromthe first and second embodiments.

A D2D Proximity Service (D2D ProSe) is a service for enabling directcommunication that is performed not via a network, in a synchronizationcluster including a plurality of synchronized user terminals. The D2DProximity Service includes a discovery process (Discovery) fordiscovering a neighboring terminal, and a communication process(Communication) for performing direct communication.

Meanwhile, in a mobile communication system, user terminals generallyperform communication based on the control (or assistance) from anetwork. The D2D Proximity Service, however, is assumed to be usableeven outside the coverage of the network. On the outside of the coverageof the network, the control (or assistance) from the network cannot beperformed. It is therefore difficult to appropriately use the D2DProximity Service. Thus, the following embodiment provides a userterminal and a method that can appropriately use the D2D ProximityService even outside the coverage of the network.

(D2D Proximity Service)

The overview of the D2D Proximity Service (D2D ProSe) will be describedbelow.

An LTE system according to the third embodiment supports the D2DProximity Service. The D2D Proximity Service is a service for enablingdirect communication that is performed not via a network, in asynchronization cluster including a plurality of synchronized userterminals. The D2D Proximity Service includes a discovery process(Discovery) for discovering a neighboring terminal, and a communicationprocess (D2D Communication) for performing direct communication. The D2Dcommunication is also referred to as direct communication.

A scenario in which all UEs 100 forming a synchronization cluster arepositioned inside the cell coverage is referred to as “inside thecoverage (in coverage)”. A scenario in which all the UEs 100 forming asynchronization cluster are positioned outside the cell coverage isreferred to as “outside the coverage (out of coverage)”. A scenario inwhich a part of UEs 100 in the synchronization cluster is positionedinside the cell coverage, and a remaining UE 100 is positioned outsidethe cell coverage is referred to as “partial coverage”.

FIG. 15 is a diagram illustrating respective scenarios on the inside andoutside of the coverage.

As illustrated in FIG. 15, on the inside of the coverage, the eNB 200serves as a D2D synchronization source. In the third embodiment, the D2Dsynchronization source refers to a node transmitting a D2Dsynchronization signal (synchronization source). In addition, a D2Dun-synchronization source refers to a node performing synchronizationwith the D2D synchronization source without transmitting a D2Dsynchronization signal (un-synchronization source).

On the inside of the coverage, the eNB 200 serving as a D2Dsynchronization source transmits, using a broadcast signal, D2D resourceinformation indicating radio resource usable for the D2D ProximityService. The D2D resource information includes information indicatingradio resource for discovery (hereinafter, referred to as “discoveryresource”) and information indicating radio resource for D2Dcommunication (hereinafter, referred to as “communication resource”).

The UE 100-1 serving as a D2D un-synchronization source performsdiscovery and D2D communication based on the D2D resource informationreceived from the eNB 200.

On the other hand, on the outside of the coverage, the eNB 200 cannotperform the above-described operation. Thus, a certain UE 100 (UE 100-2in FIG. 15) serves as a D2D synchronization source. The UE 100-2 servingas a D2D synchronization source transmits D2D resource information usinga broadcast signal. The UE 100-3 serving as a D2D un-synchronizationsource performs discovery and D2D communication based on the D2Dresource information received from the UE 100-2.

Here, the UE 100 cannot acquire D2D resource information from the eNB200 on the outside of the coverage. This leads to a problem in themethod of deciding D2D resource information to be used outside thecoverage.

As a method for deciding D2D resource information to be used outside thecoverage, a following method can be conceived. Specifically, in themethod, the UE 100 stores D2D resource information allocated from theeNB 200 on the inside of the coverage, and uses the D2D resourceinformation on the outside of the coverage.

Nevertheless, on the outside of the coverage, a plurality of UEs 100 towhich different pieces of D2D resource information are allocated bydifferent eNBs 200 is assumed to exist. For example, if UEs 100 executediscovery based on different pieces of discovery resource information,different discovery resources are used for the transmission andreception. In such a case, there is concern that the complexity ofdiscovery reception may increase, and discovery may fail. It istherefore difficult to appropriately use the D2D Proximity Service onthe outside of the coverage.

(Broadcast Synchronization Information)

Broadcast synchronization information transmitted by the D2Dsynchronization source will be described below. The broadcastsynchronization information is information necessary for performingsynchronization, discovery, and D2D communication.

In the third embodiment, the broadcast synchronization informationincludes a D2D synchronization signal (D2DSS) and a physical D2Dsynchronization channel (PD2DSCH).

The D2DSS is a signal for providing synchronization references of timeand frequency. The D2DSS may be formed so that an identifier or a typeof a D2D synchronization source can be identified. The D2DSS may includea primary D2DSS (PD2DSS) and a secondary D2DSS (SD2DSS).

The PD2DSCH is a physical channel for carrying larger amount ofinformation than the D2DSS does. The PD2DSCH carries an identifier or atype of a D2D synchronization source and the above-described D2Dresource information. Alternatively, the PD2DSCH may be unnecessitatedby associating the D2D resource information with the D2DSS.

(Operation According to Third Embodiment)

(1) Operation Overview

In the third embodiment, the UE 100 stores D2D resource informationreceived from the D2D synchronization source (the eNB 200 or the UE100). When the UE 100 becomes a D2D synchronization source on theoutside of the coverage, the UE 100 transmits the stored D2D resourceinformation by broadcast. More specifically, the UE 100 transmits theD2D resource information with the D2D resource information included inthe above-described broadcast synchronization information.

On the other hand, when the UE 100 becomes a D2D un-synchronizationsource performing synchronization with the D2D synchronization source,the UE 100 rewrites the stored D2D resource information using the D2Dresource information received from the D2D synchronization source. Morespecifically, the UE 100 rewrites the stored D2D resource informationusing the D2D resource information included in the broadcastsynchronization information received from the D2D synchronizationsource, and performs discovery (and D2D communication) based on therewritten D2D resource information.

With this configuration, even if D2D resource information stored by theUE 100 serving as a D2D un-synchronization source is different from D2Dresource information stored by another UE 100 serving as a D2Dsynchronization source, on the outside of the coverage, the UE 100 canperform discovery (and D2D communication) according to the D2D resourceinformation stored by the other UE 100 serving as a D2D synchronizationsource. For example, the UEs 100 can execute discovery based on the samediscovery resource information, and the same discovery resource can beused for the transmission and reception.

In addition, the UE 100 needs to decide whether the UE 100 itself servesas a D2D synchronization source or a D2D un-synchronization source onthe outside of the coverage.

In the third embodiment, the UE 100 receives, from another UE 100,priority information indicating the degree of suitability for a D2Dsynchronization source. By comparing the received priority informationwith predetermined information, the UE 100 determines whether to set theother UE 100 as a D2D synchronization source or set the UE 100 itself asa D2D synchronization source. The predetermined information is priorityinformation of the UE 100 itself or a reference value acquired from anetwork.

In addition, in the third embodiment, the UE 100 transmits, bybroadcast, priority information indicating the degree of suitability fora D2D synchronization source. The priority information of the UE 100itself is based on at least one of the specification of the UE 100itself, the movement state of the UE 100 itself, and the reliability ofD2D resource information stored in a storage.

FIG. 16 is a diagram illustrating an operation overview according to thethird embodiment.

As illustrated in FIG. 16, in the initial state, the UEs 100-1 to 100-3each serve as a D2D synchronization source and transmit broadcastsynchronization information. In addition, the UEs 100-1 to 100-3transmit their respective pieces of priority information with the piecesof priority information included in their respective pieces of broadcastsynchronization information.

The priority information of the UE 100-1 is “Priority 10”, the priorityinformation of the UE 100-2 is “Priority 1”, and the priorityinformation of the UE 100-3 is “Priority 5”. The UE 100-4 receives thebroadcast synchronization information from each of the UEs 100-1 to100-3.

The UE 100-4 compares the respective pieces of priority information ofthe UEs 100-1 to 100-3 with one another, and selects the UE 100 with thehighest priority. Since the priority of the UE 100-1 with “Priority 10”is the highest, the UE 100-4 selects the UE 100-1, and sets the UE 100-1as a D2D synchronization source of the UE 100 itself. In addition, inthis process, the priority information of the UE 100-4 is notconsidered.

In this case, the UE 100-4 rewrites D2D resource information stored bythe UE 100 itself, using D2D resource information included in broadcastsynchronization information of the UE 100-1. The UE 100-4 performsdiscovery (and D2D communication) according to the D2D resourceinformation included in the broadcast synchronization information of theUE 100-1.

In addition, the UEs 100-1 and 100-4 form one synchronization cluster,and the UE 100-1 serves as a synchronization source UE (synchronizationcluster head, control UE) in the synchronization cluster.

In addition, the description has been given of an example case in whichpriority information is included in broadcast synchronizationinformation. Alternatively, instead of including priority information inbroadcast synchronization information, or in addition to includingpriority information in broadcast synchronization information, priorityinformation may be included in a discovery signal.

(2) Priority Information

The priority information of the UE 100 is based on at least one of thespecification of the UE 100, the movement state of the UE 100, and thereliability of D2D resource information stored in a storage. Morespecifically, for deciding priority information, at least one of thefollowing parameters is used.

-   -   Whether a UE 100 is inside the coverage: Higher priority is        given to a UE 100 inside the coverage as compared with a UE 100        outside the coverage    -   The time when D2D resource information is acquired from a D2D        synchronization source inside the coverage (the eNB 200, the UE        100 inside the coverage): As an elapsed time from the time when        D2D resource information is acquired from a D2D synchronization        source inside the coverage is longer, lower priority is given.    -   UE ID, ProSe ID, and Application ID: As for the UE ID and the        ProSe ID, a setting of giving synchronization source priority to        a UE 100 having a specific UE ID (ProSe ID) such as, for        example, a UE of a commander of an army and a UE of a captain of        fire crews is used. In addition, as for the Application ID, when        a specific application such as, for example, Public Safety and        telephone call is planned to be used, higher priority is given        to a UE so that the UE preferentially becomes a synchronization        source.    -   Whether a UE 100 is a high power UE: Higher priority is given to        a UE 100 having large maximum transmission power.    -   The number of times a UE 100 connects to a D2D synchronization        source outside the coverage: As the number of times a UE 100        connects to a D2D synchronization source outside the coverage is        larger, the reliability of D2D resource information is        considered to be lower. Thus, lower priority is given to the UE        100. In addition, when D2D resource information is acquired from        a D2D synchronization source inside the coverage, the number is        reset.    -   Whether a UE 100 has a global positioning system (GPS): Since        the time accuracy of a UE 100 having a GPS (GNSS) is considered        to be higher, higher priority is given.    -   UE category: Higher priority is given to a UE 100 belonging to a        category with high communications capacity.    -   Supported release: As the release of a standard with which a UE        100 complies is more recent, higher priority is given.    -   Movement state (speed information): As the movement speed of a        UE 100 is higher, lower priority is given.

(3) D2D Resource Information Rewrite Determination

The UE 100 executes D2D resource information rewrite determination atone or more timings of the following timings.

-   -   A timing at which a UE 100 receives broadcast synchronization        information (D2D synchronization signal and D2D resource        information) from the eNB 200, i.e., a timing at which the E 100        comes inside the coverage.    -   A timing at which the UE 100 receives broadcast synchronization        information from another UE 100.    -   A timing at which the UE 100 receives a discovery signal from        another UE 100.

In addition, the UE 100 that serves as a D2D synchronization source andis executing D2D communication does not have to update D2D resourceinformation. In this case, the UE 100 continues to be a D2Dsynchronization source during communication. If new D2D communicationdoes not occur for a certain time period or longer after thecommunication ends, D2D resource information is updated.

(4) Operation of D2D Synchronization Source

At a timing at which transmission or reception is not performed, the UE100 serving as a D2D synchronization source may receive priorityinformation from another UE 100, and determine whether to continue to bea D2D synchronization source or stop being a D2D synchronization source(i.e., the transmission of broadcast synchronization information).

FIG. 17 is a sequence diagram illustrating an operation of the UE 100serving as a D2D synchronization source according to the thirdembodiment. In the initial state in FIG. 17, the UEs 100-1 and 100-2each serve as a D2D synchronization source and transmit broadcastsynchronization information.

As illustrated in FIG. 17, in step S501, the UE 100-2 transmits priorityinformation “Priority B” of the UE 100-2 itself. The UE 100-1 receivesthe priority information “Priority B” of the UE 100-2.

In step S502, the UE 100-1 compares the priority information “PriorityB” of the UE 100-2 with priority information “Priority A” of the UE100-1 itself. In this example, the description will be given assumingthat the priority information “Priority B” of the UE 100-2 is higherthan the priority information “Priority A” of the UE 100-1 itself.

In step S503, the UE 100-1 stops being a D2D synchronization source,i.e., stops transmitting broadcast synchronization information.

In step S504, the UE 100-1 sets the UE 100-2 as a synchronization sourceUE of the UE 100-1 itself, and performs synchronization with the UE100-2.

In addition, in this sequence, priority information of another UE iscompared with its own priority information. Nevertheless, the comparisonperformed here is not limited to the comparison with own priorityinformation. The priority information may be compared with a referencevalue acquired from a network. The reference value acquired from anetwork is a threshold value preset by an AS, an NAS, and the like, forexample.

(5) Operation of D2D Un-Synchronization Source

When D2D resource information is not acquired from a D2D synchronizationsource inside the coverage, the UE 100 serving as a D2Dun-synchronization source first searches for broadcast synchronizationinformation (and discovery signal). Alternatively, the UE 100 serving asa D2D un-synchronization source becomes a D2D synchronization sourcebefore searching, and is prohibited from transmitting broadcastsynchronization information (and discovery signal).

The UE 100 serving as a D2D un-synchronization source may determinewhether to become a D2D synchronization source, based on receivedpriority information, or when broadcast synchronization information hasfailed to be received from another UE 100.

FIG. 18 is a sequence diagram illustrating an operation of the UE 100serving as a D2D un-synchronization source according to the thirdembodiment. In the initial state in FIG. 18, the UE 100-1 serves as aD2D synchronization source, and the UE 100-2 serves as a D2Dun-synchronization source.

As illustrated in FIG. 18, in step S601, the UE 100-1 transmits priorityinformation “Priority A” of the UE 100-1 itself. The UE 100-2 receivesthe priority information “Priority A” of the UE 100-1.

In step S602, the UE 100-2 compares the priority information “PriorityA” of the UE 100-1 with priority information “Priority B” of the UE100-2 itself. In this example, the description will be given assumingthat the priority information “Priority B” of the UE 100-2 is higherthan the priority information “Priority A” of the UE 100-1.

In step S603, the UE 100-2 becomes a D2D synchronization source, i.e.,initiates transmitting broadcast synchronization information.

In step S604, the UE 100-2 transmits broadcast synchronizationinformation.

In addition, in this sequence, priority information of another UE iscompared with its own priority information. Nevertheless, the comparisonperformed here is not limited to the comparison with own priorityinformation. The priority information may be compared with a referencevalue acquired from a network. The reference value acquired from anetwork is a threshold value preset by an AS, an NAS, and the like, forexample

[Fourth Embodiment]

The fourth embodiment will be described mainly based on a differencefrom the first to third embodiments.

(Multi-Hop Synchronization Scheme)

As described above, the UE 100 serving as a D2D synchronization source,i.e., a synchronization cluster head (SCH) UE 100 provides informationnecessary for executing synchronization, discovery, and D2Dcommunication, using the D2DSS and the PD2DSCH. By receiving the D2DSStransmitted from the D2D synchronization source (SCH), a UE 100 acquirestime and frequency synchronization references. Furthermore, whenexecuting discovery and D2D communication, the UE 100 needs to recognizea resource used by another UE 100 located close to the UE 100. Thus,radio resources (resource pools) for discovery and D2D communicationneed to be provided. In addition, since the UE 100 may receive aplurality of D2DSSs transmitted from different SCHs, SCH-relatedinformation should be transmitted. As the SCH-related information, a D2Dsynchronization source identifier and a D2D synchronization source typeare considered. The D2D synchronization source should notify a UE 100 ofthese types of information using the D2DSS, the PD2DSCH, and othermeans. Table 1 lists pieces of information to be provided by the D2Dsynchronization source.

TABLE 1 Time/frequency synchronization reference Bandwidth SFNinformation D2D resource pool Discovery resource pool Communicationresource pool Synchronization source related information PD2DSCHtransmission periodicity and offset Synchronization source typeIn-Coverage Synchronization source Synchronization Cluster Head (SCH)(Out-of-Coverage) Synchronization source Synchronization source IDNumber of hop

In the fourth embodiment, a case of introducing a multi-hopsynchronization scheme will be described. The multi-hop synchronizationscheme is a scheme in which the UE 100 multi-hop transfers broadcastsynchronization information (D2DSS and PD2DSCH) acquired from the eNB200 or a SCH UE 100.

FIG. 19 is a diagram illustrating the multi-hop synchronization schemein an out-of-coverage scenario. As illustrated in FIG. 19, in each of aplurality of synchronization clusters 1 to 3, broadcast synchronizationinformation transmitted by a SCH UE is transferred by another UE in acorresponding synchronization cluster. As a result, even a UE thatcannot directly receive broadcast synchronization information from a SCHUE can be synchronized with UEs under the SCH UE, and execute discoveryand D2D communication.

FIG. 20 is a diagram illustrating the multi-hop synchronization schemein a partial-coverage scenario. As illustrated in FIG. 20, broadcastsynchronization information transmitted by an eNB 1 is transferred by aUE inside the coverage. As a result, even an out-of-coverage UE thatcannot directly receive broadcast synchronization information from theeNB 1 can be synchronized with the UE under the eNB 1, and executediscovery and D2D communication.

(Operation According to Fourth Embodiment)

(1) Operation Overview

FIG. 21 is a diagram illustrating a transfer method of broadcastsynchronization information (D2DSS and PD2DSCH) in the multi-hopsynchronization scheme.

As illustrated in FIG. 21, when broadcast synchronization information istransferred, interference is considered to occur between broadcastsynchronization information before transfer and broadcastsynchronization information after transfer. In FIG. 21, interferenceoccurs between broadcast synchronization information (hop 1) transmittedby the UE 100-1 serving as a D2D synchronization source (SCH) andbroadcast synchronization information (hop 2) transferred by the UE100-2. In addition, interference occurs between the broadcastsynchronization information (hop 2) transferred by the UE 100-2 andbroadcast synchronization information (hop 3) transferred by the UE100-3. It is therefore preferable to introduce a transfer method thatgives consideration to interference.

The transfer method of broadcast synchronization information accordingto the fourth embodiment will be described below. Initially, thedescription will be given with the focus placed on the UE 100-2.

First, the UE 100-2 receives the broadcast synchronization information(hop 1) multi-hop transferred from the D2D synchronization source (SCHUE 100-1).

Secondly, the UE 100-2 transfers transmission broadcast synchronizationinformation (hop 2) corresponding to the received broadcastsynchronization information (hop 1), to another UE 100-3. Here, the UE100-2 applies a transmission parameter different from a transmissionparameter applied to the received broadcast synchronization information(hop 1), to the transmission broadcast synchronization information (hop2). The transmission parameter is at least either one of a signalsequence or time and frequency resources.

More specifically, the transmission parameter in broadcastsynchronization information is associated with the hop number of thebroadcast synchronization information from the D2D synchronizationsource (SCH UE 100-1). In addition, the broadcast synchronizationinformation (hop 1) received by the UE 100-2 includes information aboutthe hop number from the D2D synchronization source. The UE 100-2 appliesa transmission parameter corresponding to the hop number, to thetransmission broadcast synchronization information (hop 2).

The UE 100-3 also performs operations similar to those performed by theUE 100-2. More specifically, the broadcast synchronization information(hop 2) received by the UE 100-3 includes information about the hopnumber from D2D synchronization source. The UE 100-2 applies atransmission parameter corresponding to the hop number, to thetransmission broadcast synchronization information (hop 3).

(2) D2DSS

Next, the description will be given of a specific example of atransmission parameter in broadcast synchronization information that isto be applied to a D2DSS.

One method for suppressing the interference between D2DSSs is to changea signal sequence (an orthogonal sequence) of a D2DSS for each hop, forexample Examples of the signal sequence include a Zadoff Chu sequenceand an M sequence. With this configuration, signal sequences of D2DSSstransmitted by the UEs 100-1 to 100-3 can be made different from oneanother. Thus, D2DSSs can be multiplexed by code division multiplexing.

(3) PD2DSCH

Next, the description will be given of a specific example of atransmission parameter in broadcast synchronization information that isto be applied to a PD2DSCH.

A method for suppressing the interference between PD2DSCHs is to changea transmission resource of a PD2DSCH for each hop. FIG. 22 is a diagramillustrating a method for suppressing the interference between PD2DSCHs.

As illustrated in FIG. 22, the offset of the transmission periodicity ofPD2DSCHs is changed according to the hop number. For example, thetransmission periodicity of PD2DSCHs is set to be an integer multiple ofthe transmission periodicity of D2DSSs, and the offset varying accordingto each hop number is given to the transmission timings of PD2DSCHs.With this configuration, PD2DSCH transmission timings of the UEs 100-1to 100-3 can be made different from one another. Thus, D2DSSs can bemultiplexed by time division multiplexing.

[Modified Example of Third and Fourth Embodiments]

In the fourth embodiment, the UE 100 may decide a transmission parameterto be applied to transmission broadcast synchronization information, byscanning broadcast synchronization information transmitted from anotherUE 100. For example, the UE 100 scans broadcast synchronizationinformation transmitted from another UE 100, determines a transmissionparameter of the broadcast synchronization information, and selects atransmission parameter different from the transmission parameter. Then,the UE 100 applies the selected transmission parameter to broadcastsynchronization information, and transmits the broadcast synchronizationinformation.

The above-described third and fourth embodiments may be executed incombination with the first and second embodiments.

In the above-described third and fourth embodiments, an LTE system hasbeen described as an example a mobile communication system. The presentdisclosure, however, is not limited to the LTE system. The presentdisclosure may be applied to a system other than the LTE system.

CROSS-REFERENCE

Japanese Patent Application No. 2013-202767 (filed on Sept. 27, 2013)and Japanese Patent Application No. 2014-014915 (filed on Jan. 29, 2014)are incorporated by reference herein in its entirety.

INDUSTRIAL APPLICABILITY

The present disclosure is useful in a mobile communication field.

The invention claimed is:
 1. A first user terminal comprising: acontroller containing at least one processor and at least one memory,wherein in case that the first user terminal is out of coverage area ofa communication network including a plurality of base stations, thecontroller is configured to execute processes of: directly receivingfirst device-to-device (D2D) synchronization information from a seconduser terminal being in the coverage area, the first D2D synchronizationinformation including first coverage information that indicates that thesecond user terminal is in the coverage area; directly receiving secondD2D synchronization information from a third user terminal being out ofthe coverage area, the second D2D synchronization information includingsecond coverage information that indicates that the third user terminalis out of the coverage area; determining, based on the first coverageinformation, that the second user terminal is in the coverage area;determining, based on the second coverage information, that the thirduser terminal is out of the coverage area; selecting the second userterminal as a synchronization reference of the first user terminal inresponse to determining that the second user terminal is in the coveragearea and the third user terminal is out of the coverage area; andreceiving usable resource information broadcasted from the second userterminal that becomes the synchronization reference, the usable resourceinformation indicating D2D usable resources notified from a base stationto the second user terminal by broadcast signaling.
 2. The first userterminal according to claim 1, wherein the controller is furtherconfigured to execute processes of: transmitting third D2Dsynchronization information after selecting the second user terminal asthe synchronization reference; and selecting second time resources fortransmitting the third D2D synchronization information, the second timeresources being different from first time resources for transmitting thefirst D2D synchronization information.
 3. A method for performing by afirst user terminal, comprising: in case that the first user terminal isout of coverage area of a communication network including a plurality ofbase stations, directly receiving first device-to-device (D2D)synchronization information from a second user terminal being in thecoverage area, the first D2D synchronization information including firstcoverage information that indicates that the second user terminal is inthe coverage area; directly receiving second D2D synchronizationinformation from a third user terminal being out of the coverage area,the second D2D synchronization information including second coverageinformation that indicates that the third user terminal is out of thecoverage area; determining, based on the first coverage information,that the second user terminal is in the coverage area; determining,based on the second coverage information, that the third user terminalis out of the coverage area; selecting the second user terminal as asynchronization reference of the first user terminal in response todetermining that the second user terminal is in the coverage area andthe third user terminal is out of the coverage area; and receivingusable resource information broadcasted from the second user terminalthat becomes the synchronization reference, the usable resourceinformation indicating D2D usable resources notified from a base stationto the second user terminal by broadcast signaling.
 4. The methodaccording to claim 3, further comprising: transmitting third D2Dsynchronization information after selecting the second user terminal asthe synchronization reference; and selecting second time resources fortransmitting the third D2D synchronization information, the second timeresources being different from first time resources for transmitting thefirst D2D synchronization information.
 5. A device for controlling afirst user terminal, comprising: at least one processor and at least onememory, wherein in case that the first user terminal is out of coveragearea of a communication network including a plurality of base stations,the at least one processor is configured to execute processes of:directly receiving first device-to-device (D2D) synchronizationinformation from a second user terminal being in the coverage area, thefirst D2D synchronization information including first coverageinformation that indicates that the second user terminal is in thecoverage area; directly receiving second D2D synchronization informationfrom a third user terminal being out of the coverage area, the secondD2D synchronization information including second coverage informationthat indicates that the third user terminal is out of the coverage area;determining, based on the first coverage information, that the seconduser terminal is in the coverage area; determining, based on the secondcoverage information, that the third user terminal is out of thecoverage area; selecting the second user terminal as a synchronizationreference of the first user terminal in response to determining that thesecond user terminal is in the coverage area and the third user terminalis out of the coverage area; and receiving usable resource informationbroadcasted from the second user terminal that becomes thesynchronization reference, the usable resource information indicatingD2D usable resources notified from a base station to the second userterminal by broadcast signaling.
 6. The device according to claim 5,wherein the at least one processor is further configured to executeprocesses of: transmitting third D2D synchronization information afterselecting the second user terminal as the synchronization reference; andselecting second time resources for transmitting the third D2Dsynchronization information, the second time resources being differentfrom first time resources for transmitting the first D2D synchronizationinformation.